Ionizing radiations result from the disintegration of unstable nuclei, and it is these radiations that are damaging to living tissue. The irradiation in which one is interested may originate from an external source or from radioactive materials within the body. Radiation effects from radioactive materials within the body depend upon several factors: the quantity of radioactive material absorbed in the body; the type of radiation, whether alpha, beta, gamma, etc.; the energy; the point of localization; the rate of excretion from the body; and the half-life of rate of decay of the material. All of these factors are considered in evaluating the maximum permissible concentration (MPC) for each radioisotope in water or air. The MPC values under consideration for release by the Subcommittee on Internal Dose of the National Committee on Radiation Protection are indicated.